This invention relates generally to turbine engine shroud segments and shroud segment assemblies including a surface exposed to elevated temperature engine gas flow. More particularly, it relates to air cooled gas turbine engine shroud segments, for example used in the turbine section of a gas turbine engine, and made of a low ductility material.
A plurality of gas turbine engine stationary shroud segments assembled circumferentially about an axial flow engine axis and radially outwardly about rotating blading members, for example about turbine blades, defines a part of the radial outer flowpath boundary over the blades. As has been described in various forms in the gas turbine engine art, it is desirable to maintain the operating clearance between the tips of the rotating blades and the cooperating, juxtaposed surface of the stationary shroud segments as close as possible to enhance engine operating efficiency. Typical examples of U.S. patents relating to turbine engine shrouds and such shroud clearance include U.S. Pat. No. 5,071,313xe2x80x94Nichols; U.S. Pat. No. 5,074,748xe2x80x94Hagle; U.S. Pat. No. 5,127,793xe2x80x94Walker et al.; and U.S. Pat. No. 5,562,408xe2x80x94Proctor et al.
In its function as a flowpath component, the shroud segment and assembly must be capable of meeting the design life requirements selected for use in a designed engine operating temperature and pressure environment. To enable current materials to operate effectively as a shroud in the strenuous temperature and pressure conditions as exist in the turbine section flowpath of modern gas turbine engines, it has been a practice to provide cooling air to a radially outer portion of the shroud. Examples of typical cooling arrangements are described in some of the above identified patents.
The radially inner or flow path surfaces of shroud segments in a gas turbine engine shroud assembly about radially inward rotating blades are arced circumferentially to define a flowpath annular surface about the rotating tips of the blades. Such annular surface is the sealing surface for the turbine blade tips. Since the shroud is a primary element in a turbine blade clearance control system, minimizing shroud deflection and maintaining shroud radially inner surface arc or xe2x80x9croundnessxe2x80x9d during operation of a gas turbine engine assists in minimizing performance penalty to an engine cycle. Several operating conditions tend to distort such roundness.
One condition is the application of cooling air to the radially outer portion of a shroud segment, creating in the shroud segment a thermal gradient or differential between the radially inner shroud surface exposed to a relatively high operating gas flow temperature and the cooled radially outer surface. One result of such thermal gradient is a form of shroud segment deformation or deflection generally referred to as xe2x80x9cchordingxe2x80x9d. At least the radially inner or flowpath surface of a shroud and its segments are arced circumferentially to define a flowpath annular surface about the rotating tips of the blades. The thermal gradient between the inner and outer faces of the shroud, resulting from cooling air impingement on the outer surface, causes the arc of the shroud segments to chord or tend to straighten out circumferentially. As a result of chording, the circumferential end portions of the inner surface of the shroud segment tend to move radially outwardly in respect to the middle portion of the segment.
In addition to thermal distorting forces generated by such thermal gradient are distorting fluid pressure forces, acting on the shroud segment. Such forces result from a fluid pressure differential between the higher pressure cooling air on the shroud segment radial outer surface and the axially decreasing lower pressure engine flowstream on the shroud radially inner surface. With the cooling air maintained at a substantially constant pressure on the shroud radially outer surface during engine operation, such fluid pressure differential on a shroud segment increases axially downstream through the engine in a turbine section as the turbine extracts power from the gas stream. This action reduces the flow stream pressure progressively downstream. Such pressure differential tends to force the axial end portions, more so the axially aft or downstream portion, of a shroud segment radially inwardly. Therefore, a complex array of forces and pressures act to distort and apply pressures to a turbine engine shroud segment during engine operation to change the roundness of the arced shroud segment assembly radially inner surface. It is desirable in the design of such a turbine engine shroud and shroud assembly to compensate for such forces and pressures acting to deflect or distort the shroud segment.
Metallic type materials currently and typically used as shrouds and shroud segments have mechanical properties including strength and ductility sufficiently high to enable the shrouds to be restrained against such deflection or distortion resulting from thermal gradients and pressure differential forces. Examples of such restraint include the well known side rail type of structure, or the C-clip type of sealing structure, for example described in the above identified Walker et al patent. That kind of restraint and sealing results in application of a compressive force at least to one end of the shroud to inhibit chording or other distortion.
Current gas turbine engine development has suggested, for use in higher temperature applications such as shroud segments and other components, certain materials having a higher temperature capability than the metallic type materials currently in use. However such materials, forms of which are referred to commercially as a ceramic matrix composite (CMC), have mechanical properties that must be considered during design and application of an article such as a shroud segment. For example, as discussed below, CMC type materials have relatively low tensile ductility or low strain to failure when compared with metallic materials. Also, CMC type materials have a coefficient of thermal expansion (CTE) in the range of about 1.5-5 microinch/inch/xc2x0 F., significantly different from commercial metal alloys used as restraining supports or hangers for metallic shrouds and desired to be used with CMC materials. Such metal alloys typically have a CTE in the range of about 7-10 microinch/inch/xc2x0 F. Therefore, if a CMC type cooled on one surface during operation, forces can be developed in CMC type segment sufficient to cause failure of the segment.
Generally, commercially available CMC materials include a ceramic type fiber for example SiC, forms of which are coated with a compliant material such as BN. The fibers are carried in a ceramic type matrix, one form of which is SiC. Typically, CMC type materials have a room temperature tensile ductility of no greater than about 1%, herein used to define and mean a low tensile ductility material. Generally CMC type materials have a room temperature tensile ductility in the range of about 0.4-0.7%. This is compared with metallic shroud and/or supporting structure or hanger materials having a room temperature tensile ductility of at least about 5%, for example in the range of about 5-15%. Shroud segments made from CMC type materials, although having certain higher temperature capabilities than those of a metallic type material, cannot tolerate the above described and currently used type of compressive force or similar restraint force against chording and other deflection or distortion. Neither can they withstand a stress rising type of feature, for example one provided at a relatively small bent or filleted surface area, without sustaining damage or fracture typically experienced by ceramic type materials. Furthermore, manufacture of articles from CMC materials limits the bending of the SiC fibers about such a relatively tight fillet to avoid fracture of the relatively brittle ceramic type fibers in the ceramic matrix. Provision of a shroud segment of such a low ductility material, particularly in combination or assembly with a shroud support or hanger that carries the segment without application of excessive pressure to the segment, with appropriate surfaces for sealing of edge portions from leakage thereabout, would enable advantageous use of the higher temperature capability of CMC material for that purpose.
Forms of the present invention provide a turbine engine shroud segment, for example for mounting in a shroud assembly with a shroud hanger and a method for making such a shroud. The shroud segment comprises a shroud segment body and a shroud segment projection integral with and projecting generally radially outwardly from the shroud body. The shroud segment body includes a radially inner surface; a radially outer surface; a first plurality, in one example a pair, of spaced apart axial edge surfaces connected with and between each of the inner and outer surfaces; and a second plurality, in one example a pair, of spaced apart circumferential edge surfaces connected with and between each of the inner and outer surfaces.
The shroud segment includes a shroud segment projection integral with and extending generally radially outwardly from the shroud body radially outer surface. The projection is positioned on the body radially outer surface spaced apart in a generally midway surface portion between second plurality of spaced apart circumferential edge surfaces. In one embodiment of the shroud segment in which the projection extends generally between circumferential edge surfaces, the projection is located at a position between axial edge surfaces on the body radially outer surface as a function of the fluid pressure differential experienced by the shroud segment during operation. Such location is generally at a pressure differential midpoint or balancing position between the axially forward and aft edge surfaces of the segment to reduce, and preferably substantially eliminate, during engine operation, force differences on the projection carrying the segment body. Because the pressure differential between cooling air and engine flowstream increases during operation from axially forward to aft on the segment, as power is extracted from the flowstream through a gas turbine, the projection generally is positioned niore toward the axially aft portion of the segment.
The projection comprises a projection head spaced apart from the body radially outer surface, and a projection transition portion, having a transition surface, integral with both the projection head and the midway portion of the body radially outer surface. The projection transition portion between the projection head and the body radial outer surface is smaller in cross section than the projection head, at least in one of the axial and circumferential directions. For use with a low ductility material, for example a CMC, the transition surface is arcuate to avoid a stress riser type condition in the transition portion. One embodiment of the projection integral with the body sometimes is referred to as a xe2x80x9cdovetailxe2x80x9d shape.
Another form of the present invention is a turbine engine shroud assembly comprising a plurality of the above described shroud segments, assembled circumferentially to define a segmented turbine engine shroud, and a shroud hanger carrying the shroud segments. The shroud hanger comprises a hanger radially inner surface defining a hanger cavity terminating in at least one pair of spaced apart hanger radially inner hook members opposed one to the other, each hook member including an end portion, for example as spaced apart hanger radially inner hook portions. Each end portion includes an end portion inner surface defining a portion of the hanger cavity radially inner surface and is shaped to cooperate in registry with and carry the shroud segment projection at the shroud segment projection transition surface. In one embodiment, the shroud hanger includes a shroud segment positioning member for positioning the shroud segment in at least one of the circumferential, radial and axial directions. For example, such a member is a radially inwardly positioned and preloaded pin, received at or in a recess in the projection head, applying generally radially inward pressure to the projection head sufficient to press the projection transition surfaces toward and in contact with the hanger end portion inner surfaces.